When the engine is working harder, (under a load) the turbo is working harder, putting more manifold pressure in, and forcing more air into the engine. When the engine loafs, (as in empty), the turbo is not putting out as much airflow since the engine is not burning as much fuel.

Since this air has to leave the system at some point, it flows out of the exhaust. The greater flow of air coming down the same sized pipe as the lower amount of air, comes out at a faster velocity. Essentially, you have more air flowing out of the same sized pipe.

Technically, what you hear is what we call "Air shear" which is the sound difference between the high speed flowing air, and the stationary air. As the high speed air hits the stationary air, you get air sheer where the two streams collide and that is what the exhaust noise actually is.

When you hear a large jet plane fire up and hear the noise, what you are actually hearing is the sound of the air shear between the air coming out of the engine at high speed, and the stationary air it is being pushed into and against.

And when we use the phrase "hear the wind roar", this is actually what you are hearing and referring to.

Hi Burky

thanks for the post

But what about naturally aspirated engines?
They give the same roar under load, or when loading too quickly.
Where does the extra air come from then?

An engine still pulls in air, so the sucking sound you hear is the engine. Which is why car intakes have such crazy intakes pre air filter. People hear that noise and freak out. For any other engine related questions please go here, it has interactive animated drawings:

www.howstuffworks.com

Ok, so I just won't attempt to help you understand anything. FAL, and DIAF.

Well you can't help me by answering a question I didn't ask, now can you? Sorry if it seems rude, but how would you feel if you've asked me for directions to the train station and I toled you where the hotel is?



here is a though, maybe due to more fuel being injected under load, there is more residual pressure after the explosion, and therefore more pressure in the exhaust.

The exhaust noise on my 3406e Caterpillar is from 12600 tiny explosions inside the combustion chambers every minute. Thus the saying RPM. The screech you hear from a jet engine is from the blades rotating at 21000 RPM thus causing a screeching sound when they all strain to the max.:>) and all the little bugs getting sucked into the intake and screaming for their lives.

Daannngg !!

Who are you going to believe ??....Burky and dieselsmoke ??....or "how stuff works ????"

(I was impressed with answers from the former.)